Modern optical communication systems utilize optical amplifiers to amplify wavelength division multiplexed (WDM) signal channels as they are transmitted through the system. The first optical amplifiers to be commercially deployed were Erbium doped fiber amplifiers (EDFAs), which are self-contained units placed between 60-120 km length spans of the communication link. These units contain a special Erbium doped fiber (EDF), which serves as the gain medium used to transfer energy from laser diode pumps within the units to the optical signal channels as they pass through the unit.
Recently, a new type of amplifier, namely a distributed Raman amplifier (DRA), has been introduced into optical communication systems. A significant difference between EDFAs and DRAs is that in the latter the transmission fiber itself serves as the gain medium, meaning that the signal channels are amplified as they travel through the transmission fiber. Thus, the amplification process is distributed along the transmission fiber, as opposed to being lumped in a self-contained unit as in the case of the EDFA. This allows the distance between EDFAs to be increased beyond 120 km, and/or the optical signal to noise ratio (OSNR) of the system to be improved, thus allowing higher bandwidth communication.
Distributed Raman amplifiers and their applications to optical communication systems are well known in the art, see for example, U.S. Pat. Nos. 6,519,082 and 6,631,025 and references therein. FIG. 1 shows a schematic representation of a span of an optical communication system employing distributed Raman amplifiers. These amplifiers comprise high power laser pumps 102 coupled to the amplified span in either forward configuration or backward configuration. In forward configuration the laser pump is coupled to an input 106 of a transmission fiber 108, with a pump energy 104 co-propagating with an optical signal 110. In backward configuration the laser pump is coupled to an output 112 of the transmission fiber, with a pump energy 114 counter-propagating to optical signal 110. While the shown span employs both forward and backward Raman amplifiers together, in many cases only one of the two types are employed. If the shown span is the first in the system, then it is directly preceded by an optical signal transmitter; otherwise it is preceded by an EDFA (or another type of lumped amplifier) used to amplify the signal channels from the preceding span. If the shown span is the last in the system, then it is directly followed by an optical signal receiver; otherwise it is followed by another EDFA (or another type of lumped amplifier) used to amplify the signal channels before they enter the next span.
As the pump power propagates along the transmission fiber (either in forward or backward configuration), power is transferred to the optical signal channels, thus causing them to be amplified during their propagation along the fiber. The transfer of energy occurs due to the non-linear stimulated Raman scattering effect, as described for example in “Non-linear fiber optics”, by G. P. Agrawal, pp. 316-369, Academic Press, 2nd Edition, 1995. In order for the energy transfer to be effective, the optical frequency of the pump lasers should be about 13 THz higher than that of the optical signal channels. Thus, if for example the optical signal channels occupy the communication band known as the C band (1525-1565 nm), the wavelength of the pump lasers should be in the range of 1420 nm-1480 nm. The term communication band is used to refer to the wavelength band containing the WDM signal channels transmitted by the system. The other commonly used communications band is the L band (about 1570-1610 nm). Other communication bands may be used in the future, and a single system may contain multiple communication bands. Multiple pumps with different wavelengths may be utilized to achieve uniform amplification over the entire communication band, or even over multiple bands. Furthermore, since Raman scattering is a non-linear effect, the pump power input into the transmission fiber needs to be particularly high, typically 0.2-1 W, in order for signal amplification to occur.
Thus, in contrast with EDFAs and other types of lumped amplifiers where the amplifier pump power remain contained within a closed unit, the high Raman pump power propagates along the transmission fiber, posing a potential safety hazard to persons coming into contact with the system. Particularly, if the pump is operated while a connector along the span is open, or when there is a break or cut in the transmission fiber, the pump energy may escape and cause harm to human eyes, as well as material damage to the system. As used herein, the term “open span” refers to the state where there is an open connector or fiber break or cut within the span employing the Raman amplifier, or any other situation that could cause significant leakage of pump power from the span, thus posing danger to human eyes coming in contact with the leaked power. The term “opening” is used to refer to the point along the span where the leakage of power occurs. Therefore, there is a need to immediately detect any such open span, and shut down the Raman pumps within a time span short enough to avoid harm to human eyes (henceforth referred to as “eye-safe time”). Exemplarily, International Standard IEC 60825-2, “Safety of Laser Products—Part 2: Safety of optical fiber communication systems”, may be viewed for a discussion of various aspects related to safety of laser products within fiber optic communication systems.
The automatic shutdown mechanism should on the one hand be as fail safe as possible, and on the other hand not be activated mistakenly by events that do not pose potential safety hazards. Another desired feature is that the shutdown mechanism should be an integrated feature of the Raman amplifier, to further enhance safety and to avoid dependence on other parts of the communication system. Finally, the detection system should ideally provide as much information as possible to the system management with regard to the type of failure (e.g. fiber break or open connector), and its position along the span. This facilitates rapid correction of the failure, and minimization of system downtime.
These requirements have been partly recognized in the prior art, and a number of methods and systems have been disclosed to address the problem. For example, U.S. Pat. Nos. 5,136,410 and 5,428,471 disclose monitoring the presence of optical communication signal channels in the communication band to detect an open span.. However, Raman amplification can complicate such monitoring due to significant levels of amplified spontaneous scattering (ASS) generated within the communication band by the pump energy (see for example J. Auyeung et al, “Spontaneous and Stimulated Raman scattering in long low loss fibers”, Journal of Quantum Electronics, Vol. QE-15 No. 5, P. 347, 1978). Since ASS is generated at any point where pump energy is present at a high enough level, and can propagate in both directions, it can mask the presence or absence of signal channels even when the span is open. For the purpose of this invention, ASS is divided into “in-band ASS” and “out-of-band ASS”. In-band ASS is defined to mean amplified spontaneous scattering with the communication band (or bands) used in the system. Out-of-band ASS is defined to mean amplified spontaneous scattering in a selected wavelength band that is outside the communication band (or bands) used in the system. For example, in a system using the C-band as the communication band, the selected wavelength band for out-of-band ASS may be the so-called “short band”, defined as the wavelength region 1500-1520 nm.
U.S. Pat. No. 6,683,712 addresses the issue of ASS in the communication band by providing a mechanism for monitoring the output pump energy of the Raman amplifier, and using the monitored value to estimate the ASS level. This is then used as an input to the signal detection circuit to subtract the ASS level. U.S. Pat. No. 6,373,621 discloses the use of a periodic filter to allow selective detection of the signal channels within the communication band, while blocking the ASS between the channels.
However, even if the issue of ASS in the communication band is overcome, modern communication links support Add and Drop of signal channels, which can lead to drastic changes in the signal level even if the span is still functional. In an extreme situation there may be no signal due to all channels being dropped, but the span is not open (i.e. it is still functional). This is an inherent problem with using signal channel detection as a safety mechanism.
This last problem has been addressed in U.S. Pat. No. 6,423,963, which discloses monitoring of a supervisory channel (existing in many commercial communication systems) in addition to monitoring of the signal channels. The advantage of monitoring a supervisory channel is that it should always be present during normal system operation, even if all signal channels are dropped. The disadvantage of using the supervisory channel is that it is not present in all systems, and in any case it involves relying on a feature of the system external to the Raman amplifier. However, the combined monitoring of the signal channels and the supervisory channel provides additional safety and prevents accidental shutdown when some or all of the signal channels are dropped. Another mechanism disclosed in this patent is related to the monitoring of pump energy back-reflection, which can be used to detect certain types of open connectors, but not fiber break. For example, opening a PC type connector within a certain distance of the Raman amplifier will cause a detectable increase in pump energy back-reflection. The advantage of this latter mechanism is that it is independent of system features such as the use of a supervisory channel. A main disadvantage is that it is not sensitive to certain types of open spans (e.g. fiber breaks or open APC connectors).
U.S. Pat. No. 6,807,001 also discloses the monitoring of Raman pump back-reflection to detect an open connector, in combination with monitoring of the signal channels. This invention suffers from the same disadvantages discussed above.
U.S. Pat. No. 6,519,082, assigned to the present assignee, extends the use of Raman pump back-reflection monitoring to include an analysis of the time-dependent behavior of the pump back-reflection. Such an analysis could potentially provide additional information as to the type of event triggering the change in back-reflection, and possibly allow detection of additional types of open spans. However, it would still not allow detection of all types of fiber breaks, and the time required to perform the analysis could be prohibitively long in the context of eye-safety requirements.
Another method to detect an open span is disclosed in U.S. Pat. No. 6,373,621 and US Patent Application No. 2004/0201882. In this method the Raman pump energy itself is modulated, and this modulation signal is detected at the other end of the span, with a disappearance of the modulation signal signifying an open span. If there are both forward and backward Raman amplifiers on the span, then the pump energy from both pumps can be modulated, and each amplifier will detect the modulation signal of the other. Thus, an open span will cause both Raman amplifiers to shutdown. If, as is often the case, there is only a single Raman amplifier, then it is necessary to rely on the system to send a shut down signal to the amplifier once the loss of the modulation signal has been detected at the other end of the span. In either case it is clear that the shut down mechanism is not integral to the individual Raman amplifier, and relies on other aspects of the systems.
US Pat. Application No. 2004/0090663 discloses the use of ASS outside the communication band to detect the existence of a point of high loss along the span during start-up of the amplifier pumps. As the pump power is increased during start-up, it is monitored, and when it reaches a predetermined level, a loss point is detected based on the ASS power level at the predetermined level of pump power. This invention relates only to the detection of a loss point present before start-up of the amplifier. It does not relate to continuous, dynamic monitoring of the ASS to detect in real-time an open span that occurs during operation of the amplifier. Furthermore, the invention involves monitoring of the pump power together with the ASS power, and the detection of a loss point based on a combination of both monitored powers.
Thus, there is a need for an additional protection mechanism, which does not suffer from the shortcomings described above Specifically, the mechanism should be self-contained within the Raman amplification system, and not be dependent on other features of the communication system of which the amplifier is part. Furthermore, it should be sensitive to all types of open spans, in contrast to pump energy back reflection, which is sensitive only to certain types of open connectors. A mechanism that satisfies these criteria should be based solely on the real-time detection of changes in the power level of out-of-band ASS, which is created along the transmission fiber and which propagates opposite to the direction of the pump energy (i.e. towards the amplifier) If an open span occurs within a certain distance of the Raman amplifier, then the power level of ASS reaching the amplifier will decrease, allowing the detection of the open span. Thus, such a mechanism facilitates continuous, real-time detection of an open span that occurs during operation of the amplifier.